When dividing fractions, you flip the second fraction and multiply
[tex]\dfrac{2}{7} \div \dfrac{4}{21}~becomes~\dfrac{2}{7}*\dfrac{21}{4}[/tex]
Next, we multiply:
[tex]\dfrac{2}{7}*\dfrac{21}{4}=\dfrac{42}{28} = \dfrac{21}{14}= 1 \dfrac{1}{2}
[/tex]
